Recovery from CNS injury is limited by macromolecules that accumulate in the micro-environment of damaged neurons and inhibit axonal regeneration (Berry (1982). Bibliotheca anatomica, 1-11; Ng, et al., (1996). Brain Res 720:17-24; Yiu and He, (2006). Nat Rev Neurosci 7: 617-627). In the acute phase of CNS injury, myelin-derived proteins are principally responsible for regenerative failure. The inhibitory proteins include myelin-associated glycoprotein (MAG) (Tang, et al., (1997). Mol Cell Neurosci 9:333-346), oligodendrocyte myelin glycoprotein (OMgp) (Wang, et al., (2002). Nature 417:941-944), and Nogo (Fournier, (2001) Nature 409:341-346; Filbin, (2003). Nat Rev Neurosci 4:703-713). Later in the course of CNS injury, chondroitin sulfate proteoglycans (CSPGs) in the glial scar inhibit axonal regeneration (Oohira, et al., (1991). J Neurosci 11:822-827.; Hynds and Snow, (1999). Experimental Neurology 160:244-255). MAG, OMgp, and Nogo bind to the neuronal receptors, Nogo-66 receptor (NgR1) (Fournier et al., 2001, supra) and paired immunoglobulin-like receptor B (PirB) (Atwal et al., (2008). Science 322:967-970.). MAG also binds gangliosides which might play a role in inhibition (Vyas, et al., Proc Natl Acad Sci USA. 2002 Jun. 11; 99(12): 8412-7). Co-receptors, including p75NTR (Wong et al., (2002). Nat Neurosci 5:1302-1308), Nogo-66 receptor 1 (NgR1) and LINGO1 (Mi et al., (2004). Nat Neurosci 7:221-228), are recruited into the Nogo receptor complex and neuronal signaling to RhoA is initiated (Yamashita et al., (2002). J Cell Biol 157:565-570). In certain instances, TAJ/TROY binds to NgR1 and can replace p75NTR in the p75NTR/NgR1/LINGO-1 complex to activate RhoA in the presence of myelin inhibitors (Shao, et al., Neuron (2005) 45(3):353-9). Activated RhoA causes growth cone collapse and inhibits neurite outgrowth (Kozma et al., (1997). Mol Cell Biol 17:1201-1211; Kuhn et al., (1999). J Neurosci 19:1965-1975; Madura et al., (2004). EMBO Reports 5:412-417).
Low density lipoprotein receptor-related protein-1 (LRP1) is a type-1 transmembrane receptor that binds over forty structurally and functionally distinct ligands, mediating their endocytosis and delivery to lysosomes (Strickland et al., (2002). Trends Endocrinol Metab 13:66-74). LRP1 also functions in phagocytosis of large particles, including myelin vesicles (Lillis et al., (2008). J Immunol 181:364-373; Gaultier et al., (2009). J Cell Sci 122: 1155-1162). Neurons in the CNS and PNS express LRP1 (Wolf et al., (1992). Am J Pathol 141, 37-42; Bu et al., (1994). J Biol Chem 269:18521-18528; Campana et al., (2006). J Neurosci 26: 11197-11207). At the subcellular level, LRP1 has been localized in dendritic shafts and spines, consistent with its known ability to interact with post-synaptic density proteins and regulate long-term potentiation (Brown et al., (1997). Brain Res 747:313-317.; May et al., (2004). Mol Cell Biol 24:8872-8883) and in neuronal growth cones, both in intercellular vesicles and at the cell surface (Steuble et al., (2010). Proteomics 10:3775-3788).
In neurons and neuron-like cell lines, binding and endocytosis of specific LRP1 ligands is coupled with activation of cell-signaling (Qiu et al., (2004). J Biol Chem 279:34948-34956; Hayashi et al., (2007). J Neurosci 27:1933-1941; Fuentealba et al., (2009). J Biol Chem 284:34045-34053; Mantuano, et al., (2008). J Neurosci 28:11571-11582; Shi et al., (2009). Sci Signal 2:ra18). Src family kinases (SFKs), which are activated downstream of LRP1, transactivate Trk receptors, accounting mechanistically for the ability of LRP1 ligands to induce neurite outgrowth (Shi et al., 2009, supra). However, LRP1 also regulates cell-signaling by serving as a co-receptor or by regulating the trafficking of other receptors, such as uPAR, TNFR1, and PDGF receptor (Webb et al., (2001). J Cell Biol 152:741-752; Boucher et al., (2003). Science 300:329-332; Gaultier et al., (2008). Blood 111: 5316-5325). The function of LRP1 in conjunction with other cell-signaling receptors explains the activity of LRP1 in regulation of inflammation, atherogenesis, and cell growth.
Our previous work demonstrating myelin phagocytosis by LRP1 (Gaultier et al., (2009). J Cell Sci 122: 1155-1162) prompted us to examine the role of LRP1 in pathways by which myelin-associated proteins inhibit axonal regeneration. We demonstrate that LRP1 is an endocytic receptor for myelin-associated inhibitory proteins, including e.g., MAG, OMgp, and Nogo isoforms. Binding of MAG to LRP1 recruits p75NTR into complex with LRP1. Both p75NTR and LRP1 are required for RhoA activation and inhibition of neurite outgrowth. Multiple strategies for inactivating LRP1 were effective at reversing the effects of MAG and purified myelin on neurite outgrowth. Our results suggest that LRP1 is essential for inhibitory myelin signaling. LRP1 emerges as a possible target for neutralizing inhibitory myelin-associated inhibitory proteins in the injured CNS.